Matrix metalloproteinases (MMPs) are endopeptidase-like enzymes involved in the degradation of the extracellular matrix. As such, they are involved in many physiological tissue-remodelling processes and play an important role in many biological processes, such as cell migration, bone resorption and regulation of angiogenesis. When there is an inadequate regulation of matrix metalloproteinases production or action, MMPs are implicated in the progression of certain diseases, such as cancer and degenerative diseases.

In 1962, Jerome Gross and Charles M. Lapiere were working on the rapid dissolution of organic masses such as the tail, gills and intestine during natural tadpole metamorphosis. They then found that culturing tadpole tissues on reconstituted calf collagen gels led to the degradation of the collagen peptide substrate. Their demonstration of a diffuse collagenolysis factor in living amphibian tissue, capable of degrading fragments of collagen agent, suggests that collagenolysis enzyme systems do exist in some animal tissues (1).

Continuing the work on amphibians, Nagai (2), accompanied by Lapierre and Gross, was able to isolate a collagenase enzyme: it was the first member of the MMP family: the MMP1.

Since then, we know that in humans, there are 23 of them (28 in vertebrates), which differ in their substrate specificity and are therefore divided into several subgroups:

• matrilysins (MMP7 and MMP26)
• interstitial collagenases (MMP1, MMP9 and MMP13)
• strimelysines (MMP3, MMP10, MMP11)
• gelatinases (MMP2, MPP9)
• membrane-type metalloproteinases MT-MMPs (MMP14, MMP15, MMP16, MMP17, MMP24, MMP25), which contain, unlike the other MMPs,  a transmembrane domain that anchors them to the cell surface
• others (MMP12, MMP18, MMP19, MMP21, MMP22, MMP23, MMP27 and MMP28) that cannot be classified in any of the previous groups.

MMP activity is tightly controlled at three different levels: gene transcription, proenzyme activation, and inhibition of enzymatic activity. Due to the presence of regulatory sequences in the promoter region of MMP genes, a number of external stimuli will induce or repress their expression, for example growth factors, cytokines, oncogene products, hormones, matrix components. With the exception of MMP-11 and MT-MMPs which are activated intracellularly by furin at the Golgi apparatus, the MMPs are secreted as inactive proenzymes, and the catalytic cleavage of their prodomain is necessary for their activation.

The MMP family also shares a common core structure:

• The sequence of MMPs begins with about twenty residues, generally hydrophobic, which allow the synthesized polypeptide to be directed to the secretion pathways.

• The prodomain is composed of about 80 amino acids, characterized by the presence of a PRUCUG(V/N)PD sequence strictly conserved in most MMPs. This sequence contains a cysteine residue with a thiol group at the carboxyl end.

• The catalytic domain, composed of about 170 residues, is characterized by a binding site for the catalytic zinc atom UHUEXGUHUXXGXXUH. In addition to the zinc atom of the catalytic site, the catalytic domain has several structural metal ions: a non-exchangeable zinc atom and two or three calciums, which are necessary for the stability and activity of the enzyme. Remarkably, in the catalytic domain of catalytic domain of MMP-2 and -9 (Gelatinases A and B) the presence of an insertion of 175 residues responsible for their specificity in degrading gelatin.

The thiol group of the cysteine at the carboxyl end of the pro-domain maintains the proMMPs in an inactive form. Displacement of the pro-domain by proteolysis disrupts the cysteine-zinc coupling and the thiol group is replaced by water. The enzyme can then cleave the peptide bonds of its substrates. This mechanism is famously known as "cystein switch" and was first described in 1990 by Van Wart and Birkedal-Hansen (3).

• a linker peptide or hinge region of variable length, that links the catalytic domain of MMPs to their C-terminal domain, except in the case of matrilysins (MMP-7 and MMP-26), whose primary structures consist only of the prodomain and the catalytic domain. This region is involved in intermolecular interactions initiated by MMPs that involve both the catalytic domain and the hemopexin domain.

• a hemopexin domain of about 210 amino acids at the C-terminus of the hinge peptide. This ellipsoidal disk-like domain, strongly similar to members of the hemopexin family, resembles a 4-bladed helix, with each blade formed by 4 anti-parallel β-strands and an α-helix (4). 

Given the ability of MMPs to degrade both the extracellular matrix - to allow cell migration and remodelling of the microenvironment - and the basement membrane, their possible involvement in these processes of intervention and progression of various diseases has been extensively studied.

Nevertheless, metalloproteinases pose several major problems in their physiological and pathological study.

The misunderstanding of the action of MMPs may stem first from the origin of their names; the names of metalloproteinases were indeed given at the time according to the first substrate observed during the experiment that led to their discovery.

But they do not seem to be specific to a single substrate. Far from it. Since because the matrix is not only a mechanical support for cells and an inert barrier for cells but serve as a reservoir for proteins important for cell messaging. MMPs have so the ability to degrade components external to the matrix, such as α1-antitrypsin, IL-1β, Pro-IL-1β, CXCL5, IL-8, SDF-1, latent TGF-β, latent TNF-α, IL-2Rα, IGFBP-1, VEGF, and even other MMPs which greatly complicates the game! In addition, there is some overlap, as several metalloproteinases can degrade the same substrate, with or without the intervention of certain cofactors.

Also, to say for instance that MMP9 degrades only gelatin would be a famous shortcut when at least a dozen different substrates have already been suggested since its discovery.

The second main issue arises when, in order to better define the physiological importance of the different MMPs, transgenic KO mice for the genes in question have been developed. In fact, less severe phenotypes than expected have emerged from these experiments insofar as blocking one MMP may lead to a compensatory increase in the expression of another MMP, the interdependencies between metalloproteinases being much more important than expected (5).

Thirdly, mutation of the homologous gene in different species suggests a functional distinction between humans and mice for the same MMP, such as MMP14 or MMP2 leading to the MONA spectrum syndromes in humans (Torg-Winchester and nodulosis-arthropathy-osteolysis syndrome)

Finally MMPs can play opposite roles depending on whether we are interested in physiological or pathological phenomena. In the same pathology, such as cancer, this notion even applies to MMPs with similar biochemical specificities, such as the various collagenases.

(1) Gross J., Lapierre С.M. Collagenolytic activity in amphibian tissues: a tissue culture assay. Proc Natl Acad Sci U S A. 1962;48(6):1014–22. https://doi.org/10.1073/pnas.48.6.1014.

(2) Nagai Y, Lapiere CM, Gross J. Tadpole collagenase. Preparation and purification. Biochemistry 5: 3123–3130, 1966

DOI: 10.1021/bi00874a007

(3) Van Wart HE, Birkedal-Hansen H. The cysteine switch: a principle of regulation of metalloproteinase activity with potential applicability to the entire matrix metalloproteinase gene family. Proc Natl Acad Sci U S A. 1990 Jul;87(14):5578-82. doi: 10.1073/pnas.87.14.5578. PMID: 2164689; PMCID: PMC54368.

(4) F.X. Gomis-Rüth, U. Gohlke, M. Betz, V. Knäuper, G. Murphy, C. López-Otı́n, W. Bode. The Helping Hand of Collagenase-3 (MMP-13): 2.7 Å Crystal Structure of its C-terminal Haemopexin-like Domain, Journal of Molecular Biology, Volume 264, Issue 3, 1996, Pages 556-566, ISSN 0022-2836, https://doi.org/10.1006/jmbi.1996.0661.

(5) Esparza J, Kruse M, Lee J, Michaud M, Madri JA. MMP-2 null mice exhibit an early onset and severe experimental autoimmune encephalomyelitis due to an increase in MMP-9 expression and activity. FASEB J. 2004 Nov;18(14):1682-91. doi: 10.1096/fj.04-2445com. PMID: 15522913.